1. Field of the Invention
The invention relates to condensation sensors and, in particular, to a sensor for a tape-drive device in a computer library system.
2. Description of the Related Art
It is often a problem that atmospheric water vapor condenses on mechanical or electrical equipment causing corrosion of metal parts. This can occur when air that contains water vapor comes into contact with metal structures that are at a relatively low temperature. Accordingly, equipment exposed to changes in temperature, as well as changes in air pressure, can degrade and ultimately fail as a result of unwanted condensation.
In large computer systems, data storage is often provided by means of cartridges and tape drives. These devices are used within an environment of controlled temperature and humidity to decrease the potential for performance degradation and failure due to corrosion. However, the systems that control temperature and humidity are themselves subject to failure; therefore, it is desirable in critical computer systems to provide some form of backup humidity-sensing capability.
Devices capable of sensing the presence of condensation are known in the art. A typical prior-art sensor, including an alarm, is illustrated in FIG. 1. Such a device 20 consists of a condensation sensor 22 and an alarm circuit 24. The sensor 22 typically comprises thin conductive strips 26A, 26B attached to an insulator surface 28 and positioned so as to define a gap 30 therebetween. The thickness of the conductive strips 26A, 26B is shown greatly exaggerated in FIG. 1 for the purpose of illustrating the structure of the sensor 22, but in a typical prior-art sensor their thickness is arbitrary and plays no appreciable role in the operation of the sensor, so long as a gap is created between them for receiving condensation formed in the environment around the sensor.
It is ordinarily desirable to construct the sensor 22 as small and as thin as possible, so as to not interfere with other structures in close proximity to the sensor. In operation, a first voltage is applied to the conductive strip 26A and a second voltage to the conductive strip 26B, thereby creating a potential difference between them. As a result, if sufficient moisture condenses to fill the gap 30 between the conductive strips 26A and 26B, a current flow is established between them. The alarm circuit 24 is adapted to respond to the current by setting off an alarm. The sensor 20 may be attached to any surface where moisture is undesirable.
While the prior-art sensor 22 of FIG. 1 is generally satisfactory for its intended purpose, several problems with its performance limit the scope of its usefulness. The first problem is that the alarm is set off only after the onset of condensation on the surface of the sensor. Therefore, when the alarm is triggered, condensation may already have occurred at other sensitive parts of the device being protected. Moisture can condense on a surface of interest in a device, such as the head of a drive, before it condenses on the sensor 22 if that device is colder than the sensor. Similarly, moisture can condense on another surface at the same time as it condenses on the sensor 22 if the two surfaces are at the same temperature. In either case, the condensation sensor 20 of the prior art at best provides a simultaneous indicator of condensation, which may not be sufficient to properly protect the device by avoiding condensation altogether.
This problem is particularly problematic in the context of tape drives and libraries in large computer systems, such as mainframe computers, that are sensitive to all environmental conditions. Accordingly, these systems are normally situated in computer rooms with environmental-control systems whereby temperature and humidity can be set and maintained precisely. However, as a result of human error or of malfunction of the control systems, environmental conditions that produce condensation on tape drives and other sensitive components can and often do still occur.
In theory, once a condensation sensor of the type shown in FIG. 1 detects moisture and the alarm is set off, the environmental system can be adjusted to prevent condensation. However, by the time the alarm is set off, condensation is likely to have already occurred on surfaces that are at the same temperature of or colder than the sensor, as mentioned above. In addition, further condensation continues to form until the environmental conditions have actually changed to safe levels. This lag time can be significant, especially if the adjustment of environmental parameters is performed manually by an operator who may be slow to respond to a warning because of inadvertence or error.
Moreover, even after the environmental conditions have changed, it may take considerable additional time before the moisture that has already condensed actually evaporates. When the air temperature is only a few degrees above the dew point (that is, when the air is only slightly less humid than at saturation), the rate of evaporation is very slow. Thus, another limitation of prior-art sensors is that condensation can remain on sensitive surfaces for significant periods of time after detection within a protected environment. Since the presence of even small amounts of moisture for limited time periods can be undesirable, this is a significant disadvantage.
A second problem with prior-art sensors of the type illustrated in FIG. 1 in the context of tape-drive protection is that for best results the sensor needs to be mounted on or near the head of the tape drive. This may not always be possible because tape drives are complex mechanisms with many components and, at the very least, the process of adding a sensor complicates the design of the drive. Therefore, condensation sensors are typically installed on stationary structures in the vicinity of the protected device, often at a sight that may or may not reflect the environmental conditions of the device of interest.
Accordingly, there is still a need for a condensation sensor that is capable of detecting when the environment of a protected device approaches the conditions required for condensation of air moisture, so that a warning in advance of the actual onset of condensation can be provided. In addition, there is a need for a condensation sensor that can detect the potential for condensation at the sight of a device of interest without having to be mounted on the device itself.